1. Field of the Invention
The present invention concerns the field of gas turbine engines and is directed to means for controlling the circulation of air between two enclosures inside the engine, the relative pressure between the two enclosures varying as a function of the operating conditions.
2. Description of the Related Art
A gas turbine engine comprises at least three parts: an air compressor, a combustion chamber and a turbine, the compressor feeding the combustion chamber, which produces hot gases driving the turbine. The turbine is connected to the compressor by a shaft through which it drives the latter. The engine can comprise a number of spools each with a rotor formed of a compressor, a turbine and a shaft mechanically connecting them. In the aeronautical field engines generally have two or three spools. They therefore comprise at least one rotary spool using a low-pressure (LP) drive fluid and one rotary spool using a high-pressure (HP) drive fluid, the two spools being mechanically independent of each other and turning at different speeds.
The search for ever higher efficiency leads to the development for the same engine of low-pressure turbines the average radius of which increases in particular relative to that of the high-pressure turbine, with the aim of reducing the aerodynamic load. There follows the necessity of providing a transition conduit of appropriate geometry between the stages of the high-pressure turbine and the inlet of the low-pressure turbine. This transition conduit remains relatively short because of the aeronautical application of the engine. Such conduits impose on the gases that travel through them a large deflection over a short distance, and therefore have high slopes and high diffusion. To conserve satisfactory flow quality in the swan-neck formed by the transition channel, means for blowing air along the exterior wall of the stream are provided, to avoid thickening and even separation of the boundary layer. The present applicant has developed a solution related to this problem. It is described in patent application FR 0654139 in the name of the present applicant. An enclosure for distribution of blowing fluid is provided between the exterior wall of the transition channel and an element of the turbine casing. The enclosure communicates via a fluid feed orifice with an intake area upstream of the transition channel. This intake area is preferably in the compressor so that the air injected forms a film for thermal protection of the wall.
Moreover, upstream of this transition channel, the annular stream of driving gas is delimited externally by a stator ring. The clearance between the tips of the blades of the HP turbine and the internal face of this ring is kept as small as possible, in all operating phases of the engine, because the efficiency of the turbine depends on it. The HP rotor and stator combination being subjected in operation to different relative radial and axial displacements, there follows a variation of the clearance, which has to be controlled. Air taken from the upstream end of the engine, in the compressor, is used for this purpose to ventilate the stator ring support and to control its expansion as a function of the operating conditions. The air circulating in the ventilation enclosure is then evacuated in the stream. This is known in itself. Note that the control function entails non-continuous circulation of ventilation air. This flow of air is reduced and interrupted, in particular when the operating conditions have stabilized.
If the engine comprises both such means for controlling expansion of the turbine stator ring with a flow of ventilation air circulating in a ventilation enclosure and, immediately downstream thereof, a blowing air distribution enclosure formed around the wall of the transition channel, it would be desirable to use that ventilation air as at least part of the blowing air for the exterior wall of the stream in the transition channel. However, in operation, the differential pressure between said ventilation enclosure and the blowing air distribution enclosure may change. Thus if the circulation of ventilation air is interrupted or reduced, the pressure in the ventilation enclosure falls below that of the distribution enclosure. If there were communication between the two enclosures, an unwanted reflow of gas from the distribution enclosure would occur, interfering with control of the clearance between the stator ring and the tips of the turbine blades.